Excise the tougher bit of cauliflower stem. Chop the rest of it and blanch it.
Put it to boil in fresh water, gently, with the quartered onion. Boil the potatoes in another pan. When the cauliflower and onion are soft put them through a processor. When the potato is ready put it through a mouli. Put them together in a pan to cook with the milk. Don’t let it boil over. Cook for an age. A